Such an image processing system is already known in the art, e.g. from the article "Adaptive Coding of Monochrome and Color Images", by Wen-Hsiung Chen et al, published in the IEEE transactions on communications, Vol. COM-25, No. 11, November 1977, pp. 1285 to 1292.
Therein the transform circuit performs a Discrete Cosine Transform and thereby provides for each block of image elements a matrix of coded coefficients which are function of the frequency content of the image block. The coefficient encoder used is a so-called zonal encoder which uses less bits for encoding coefficients representing the higher frequency content of the image block than for encoding coefficients representing the lower frequency content thereof, in accordance with the fact that higher frequency coefficients have statistically lower variances and vice-versa.
As is well known in the art, zonal coding has a relatively low sensitivity to errors occurring in the transmission of the compressed code coefficients to the receiver. However, to reach a predetermined quality of a reconstructed image in the receiver it requires the use of a compressed code having a number of bits which is larger than that which would be required if use were made of entropy coding to code the coefficients, i.e. if compressed codes were used whose number of bits is function of the probability of occurrence of these coefficients. Indeed, it is also well known in the art that entropy encoding allows the use of codes with maximum compression. However, it has the disadvantage of being very sensitive to coding errors.